The present invention relates to components useful in making electrical connections to microelectronic elements such as semiconductor chips, and to methods of manufacturing such components.
FIG. 1 shows a specific semiconductor package marketed by the present assignee under the registered trademark .mu.BGA. In short, the figure shows a semiconductor chip having contacts 110 on a face surface 120. A flexible, dielectric substrate 130 overlies and is juxtaposed with the face surface 120 of the chip 100. The substrate 130 has a first surface 150 facing away from the chip 100, and a second surface 160, facing towards the chip 100. The substrate 130 further has conductive leads 170 running along the second surface 160. Each lead has a connection section 180 extending across a bonding window 190. The connection sections 180 are releasably attached to the substrate, as described in U.S. Pat. Nos. 5,489,749 and 5,629,239 hereby incorporated by reference herein. Each lead terminates in a conductive terminal 200 which is exposed at or overlies the first surface 150 of the substrate 130. A compliant layer 140 is disposed between the substrate 130 and the face surface 120 of the chip 100. In this embodiment, the compliant layer 140 takes the form of a plurality of compliant pads 140, as described in U.S. Pat. No. 5,659,952 hereby incorporated by reference herein. The leads are detached and bonded to respective chip contacts 110, such as by thermocompression or ultrasonic bonding techniques. The bond windows are then covered by a coverlay or soldermask (not shown) and a liquid encapsulant 210 is dispensed or injected into and between the substrate 130 and the chip 100, as described more fully in U.S. patent application Ser. No. 08/726,697 incorporated by reference herein. The encapsulant 210 is then cured and solder balls 220 are connected to the terminals 200 and the package is cut to size, as shown, for future connection to a printed wiring board ("PWB").
As the chip 100 heats up and cools down in operation, it expands and contracts at an inherent rate, called its coefficient of expansion ("CTE"). The CTE of silicon is approximately 2.7 to 3.0 parts per million per degree Celsius. As the chip heats up and cools down in operation, the underlying PWB also heats up and cools down (and thereby expands and contracts) in response to the chip 100. However, the CTE of standard epoxy/fiberglass PWBs is approximately 15 to 20 parts per million per degree Celsius causing a mismatch in the expansion and contraction between the chip and the PWB. The construction of the package shown in FIG. 1 mechanically decouples the chip from the underlying PWB to allow for movement of the terminals 200/solder balls 220 with respect to the chip contacts 110 thereby allowing the chip to expand and contract at one rate and the PWB to expand and contract at another rate without the mechanical connection fatiguing and becoming unreliable due to the thermal mismatch, as explained more fully in U.S. Pat. No. 5,679,977 hereby incorporated by reference herein. In this type of chip package construction, it is typically better, from a reliability stand-point, for the connection section 180 of the leads 170 to have a gradual radius of curvature both at the heel of the connection section 180 (near where it connects to the chip contact 110) and at the shoulder of the connection section 180 (near where it connects to the substrate 130). Such a gradual radius of curvature better ensures that the connection section 180 does not hinge about a single point or area as it is flexing in response to the thermal mismatch phenomenon described above. To create such a gradual radius of curvature for the connection sections 180, the connection sections 180 must be a certain minimum length which depends on the stand-off (or vertical height) from the face surface 120 of the chip 100 to the second surface 160 of the substrate 130. This connection section 180 minimum length also sets the allowable minimum dimension (D.sub.W) for the bonding window 190. Likewise, the minimum dimension D.sub.W for the bonding window 190 limits the amount of area (D.sub.S) on the first surface 150 of the substrate 130 that is available for terminals 200. Typically, the preferred terminal 200 center to center distance (or "pitch") is somewhere between 0.8 and 0.65 millimeters with a typical terminal size of 275 .mu.m to 300 .mu.m and a solder ball 220 diameter of approximately 300 .mu.m to 350 .mu.m. On some package designs, it is permissible to use a terminal pitch of 0.5 millimeters which allows more terminals to be placed in the same substrate area D.sub.S. The major constraint on using a pitch of 0.5 millimeters or below is that most inexpensive PWYBs cannot be wired to receive contacts at such a fine pitch so more expensive substrates must be used.
FIGS. 2 and 3 show fragmentary side views of a chip package similar to the package shown in FIG. 1. In FIG. 2, the length of the bonding window 190a and therefore the connection section 180a have been shortened to D.sub.W1, when compared to the bonding window 190b (D.sub.W2) and connection section 180b of FIG. 3. As explained above, this shorter connection section 180a has a larger radius of curvature at the heel and shoulder of the connection section 180a and may also have a less reliable response to thermal cycling. In many packaging applications, the shorter connection section shown in FIG. 2 may have acceptable reliability characteristics. However, it is preferred to have the longer connection section 180b of the type shown in FIG. 3 because it has been found that such leads are more reliable over a longer period of time and/or under greater thermal mismatch conditions.
Another issue with respect to a larger bonding window 190b (D.sub.W2) is that the bonding window 190 may get in the way of the locations of the terminals 200 as the semiconductor manufacturer shrinks the size of its chips which causes the locations of the chip contacts to move. Semiconductor manufacturers fabricate a plurality of semiconductor chips en mass in a unitary, planar structure called a wafer. The chips are typically separated from each other later prior to the packaging operation. Typically, a semiconductor manufacturer will endeavor to take its original design for a particular semiconductor chip 100 and change it so that the same or better functionality are placed into a smaller first surface 120 area. This allows the semiconductor manufacturer to fit more chips into a single wafer. Since the cost of processing wafers is approximately constant, this process of reducing the chip size provides more chips for less manufacturing cost per chip. As a general rule of thumb, most chip shrinks are about 20% (or more) smaller than the size of the original chip and typically the chip manufacturers only perform two or three chip shrinks before discontinuing a particular chip type in favor of a new design or technology; although, some chip makers perform more chip shrinks before going to a completely new chip design depending on the aggressiveness of the chip maker and the type of die.
As the chip shrinks and the chip contacts move inward from their original locations, the bonding windows 190 and connection sections 180 in the chip package shown in FIG. 1 also have to move to compensate for the chip contact 110 movement. If the bonding windows need to be moved into the area D.sub.S on the first surface 150 of the substrate 130 that is occupied by the terminals 200, the terminal pitch and arrangement may no longer be uniform with the last version of the chip package. This causes the PWB manufacturer to have to re-design the PWB to account for the movement of the terminals 200/solder balls 220. One method of compensating for the problems that are encountered because of a chip shrink is to place the chip face up on the supporting substrate so that the chip contacts face away from the package terminals and are connected to the substrate terminals by standard wire-bonds and routing leads on a surface of the substrate. Such a face-up package variation has many useful attributes (as are described in U.S. patent application Ser. No. 08/962,988 incorporated by reference herein); however, the resulting package is generally larger and thicker than the package shown in FIG. 1.
Therefore, while the above mentioned package designs are important improvements to packaging technology, it is desirable to find a method of shortening the bonding window 190 and/or otherwise compensating for the problems encountered because of chip shrink without negatively affecting the reliability of the connection sections 180 of the leads 170 or the pitch and arrangement of the exterior package terminals.